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From: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
To: Christian Brauner <brauner@kernel.org>, Shuah Khan <shuah@kernel.org>
Cc: Kees Cook <kees@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Jan Kara <jack@suse.cz>, Oleg Nesterov <oleg@redhat.com>,
	Aleksa Sarai <cyphar@cyphar.com>,
	Andrei Vagin <avagin@google.com>, Kirill Tkhai <tkhai@ya.ru>,
	Alexander Mikhalitsyn <alexander@mihalicyn.com>,
	Adrian Reber <areber@redhat.com>,
	Pavel Tikhomirov <ptikhomirov@virtuozzo.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-kselftest@vger.kernel.org
Subject: [PATCH v3 1/4] pid_namespace: avoid optimization of accesses to ->child_reaper
Date: Tue, 24 Feb 2026 17:47:52 +0100	[thread overview]
Message-ID: <20260224164852.306583-2-ptikhomirov@virtuozzo.com> (raw)
In-Reply-To: <20260224164852.306583-1-ptikhomirov@virtuozzo.com>

To avoid potential problems related to cpu/compiler optimizations around
->child_reaper, let's use WRITE_ONCE (additional to task_list lock)
everywhere we write it and use READ_ONCE where we read it without
explicit lock. Note: It also pairs with existing READ_ONCE with no lock
in nsfs_fh_to_dentry().

Also let's add ASSERT_EXCLUSIVE_WRITER before write to identify to KCSAN
that we don't expect any concurrent ->child_reaper modifications, and
those must be detected.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
--
v3: Split from main commit. Add ASSERT_EXCLUSIVE_WRITER.
---
 kernel/exit.c | 3 ++-
 kernel/fork.c | 5 ++++-
 kernel/pid.c  | 2 +-
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 8a87021211ae..8e5e523dcc79 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -608,7 +608,8 @@ static struct task_struct *find_child_reaper(struct task_struct *father,
 
 	reaper = find_alive_thread(father);
 	if (reaper) {
-		pid_ns->child_reaper = reaper;
+		ASSERT_EXCLUSIVE_WRITER(pid_ns->child_reaper);
+		WRITE_ONCE(pid_ns->child_reaper, reaper);
 		return reaper;
 	}
 
diff --git a/kernel/fork.c b/kernel/fork.c
index e832da9d15a4..9ce2d12ec701 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2423,7 +2423,10 @@ __latent_entropy struct task_struct *copy_process(
 			init_task_pid(p, PIDTYPE_SID, task_session(current));
 
 			if (is_child_reaper(pid)) {
-				ns_of_pid(pid)->child_reaper = p;
+				struct pid_namespace *ns = ns_of_pid(pid);
+
+				ASSERT_EXCLUSIVE_WRITER(ns->child_reaper);
+				WRITE_ONCE(ns->child_reaper, p);
 				p->signal->flags |= SIGNAL_UNKILLABLE;
 			}
 			p->signal->shared_pending.signal = delayed.signal;
diff --git a/kernel/pid.c b/kernel/pid.c
index 3b96571d0fe6..76c2744493e2 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -219,7 +219,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid,
 			 * Also fail if a PID != 1 is requested and
 			 * no PID 1 exists.
 			 */
-			if (tid != 1 && !tmp->child_reaper)
+			if (tid != 1 && !READ_ONCE(tmp->child_reaper))
 				goto out_abort;
 			retval = -EPERM;
 			if (!checkpoint_restore_ns_capable(tmp->user_ns))
-- 
2.53.0



  reply	other threads:[~2026-02-24 16:49 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-02-24 16:47 [PATCH v3 0/4] pid_namespace: make init creation more flexible Pavel Tikhomirov
2026-02-24 16:47 ` Pavel Tikhomirov [this message]
2026-02-24 16:47 ` [PATCH v3 2/4] pid: check init is created first after idr alloc Pavel Tikhomirov
2026-02-24 16:47 ` [PATCH v3 3/4] pid_namespace: allow opening pid_for_children before init was created Pavel Tikhomirov
2026-02-24 16:47 ` [PATCH v3 4/4] selftests: Add tests for creating pidns init via setns Pavel Tikhomirov

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